1. Field of the Invention
The present invention relates to a twin wire former for the production of a fiber web, specifically a paper, cardboard or tissue web, from a fiber suspension. This type of twin wire former is generally referred to as xe2x80x9cRoll-Blade-Formerxe2x80x9d in the industry.
2. Description of the Related Art
A twin wire former of this type for the production of a paper web, specifically a fine paper web, is already known from the PCT-disclosure document WO 97/47803. The disclosed twin wire former includes an upstream headbox with several separation elements in its headbox nozzle, and a forming roll, preferably a suction type forming roll, having a roll diameter of xe2x89xa71.4 m and an angle of wrap of  less than 25xc2x0. In a curved twin wire zone located downstream from the forming roll, there are also methods for the introduction of pulsating pressure effects into the paper web that is being formed.
Further, a twin wire former as mentioned above, for the production of a paper web, specifically SC paper, is also known from the European patent application EP 0 627 523 A1. Here, initial dewatering of a fiber suspension occurs on a first forming roll in a forming zone. The fiber suspension is then brought onto a curved forming shoe, having a radius of 2 m to 8 m and is further dewatered. Subsequently, at least one dewatering unit including dewatering methods is located in the line. At the end zone of the twin wire zone there is a second forming roll including at least one suction zone, where the top wire of the twin wire former is separated from the forming paper web and is led away by way of a guide roll.
The two aforementioned twin wire formers have in common that the dewatering accomplished on the forming roll or in the area of the forming zone is greater than 70%. Since considerable portions of the paper web are formed without the presence of pressure pulsations, a forming quality that is only average is unavoidable when running fiber stock suspensions that are difficult to form. It is also a disadvantage that both twin wire formers have a very long open jet distance (distance: headbox nozzle to jet impact point), for example longer than 400 mm. This has a negative effect on the web quality, in machine direction (MD) as well as in machine cross direction (CD).
In order to achieve optimum sheet quality, a certain level of forming strip dewatering is particularly important. This requires very precise dimensioning of the forming angle, since large volumes are dewatered per angle degree. The optimum forming roll wrap must generally be determined during pilot trials, which are expensive and time intensive. Since the angle of wrap must always be matched to paper type, web weight and machine speed, even a small change in any one parameter causes extensive effects, which then will have to be neutralized at great expense.
If the sheet formation system is required to accommodate a larger weight range (specific production volume P), which is always the case with production lines, then the operating point abandons the optimum operating range on product changes. In the instance of the aforementioned twin wire formers, the fiber stock suspension throughput through the headbox must then be increased detrimentally in order to regain the optimum operating window.
The present invention provides an improved twin wire former to such an extent that the aforementioned disadvantages of the state of the art are avoided. A second objective is that fiber stock suspensions having a high long fiber content which makes them particularly difficult to form, for example papers, may find optimum use.
Characteristics of the present invention include:
the rotating forming roll has an open volume (storage volume) and is a non-suction type,
the rotating forming roll has a roll diameter of less than 1,400 mm,
the rotating forming roll has an angle of wrap of less than 7xc2x0,
a forming suction box is located immediately downstream from the rotating forming roll as viewed in the direction of wire travel, and
in the area of the wedge-shaped inlet nip, the fiber suspension has a stock consistency of between 0.4% and 2.0%, preferably between 0.6% and 1.5%.
By combining these characteristics in a twin wire former, the initial dewatering (dwell time) on the forming roll, or the dewatering volume is reduced to a minimum, whereby the minimum is smaller than 30% relative to the headbox throughput of a fiber stock suspension having a stock density of between 0.4% and 2.0%, preferably between 0.6% and 1.5% in the area of the wedge-shaped inlet nip. This is achieved by the maximum forming roll diameter of 1,400 mm and by the maximum forming roll angle of wrap of 7xc2x0. The maximum forming roll diameter of 1,400 mm and the maximum forming angle of wrap of 70 cause a greatly reduced dwell time on the forming roll.
Moreover, the minimum initial dewatering on the forming roll ensures a non-critical positioning of the headbox jet.
The headbox in whose nozzlexe2x80x94at least one machine-wide separation element, specifically a platexe2x80x94is located produces a high quality headbox jet. In accordance with the present invention, this allows and even favors utilization in the twin wire former, of fiber stock suspensions having a high long fiber content (for example paper) which are particularly difficult to form.
The surface of the forming roll having the xe2x80x9copen volumexe2x80x9d is grooved and/or drilled and/or deflected, or is constructed in a honeycomb design. These configurations are cost effective to produce and do not influence the rigidity or the operational safety of the forming roll negatively, which, depending upon the application may be up to 10 m wide.
In order to considerably increase the dewatering capacity of the twin wire former of the present invention, at least one additional forming suction box must be located following the forming suction box as viewed in direction of wire travel.
In order to achieve as symmetrical a web quality as possible, the forming suction boxes are located opposite each other, whereby the forming suction boxes, as viewed in the direction of wire travel, may have some distance between them.
Under technological and qualitative aspects it is advantageous if the at least one forming suction box has a curved suction surface having a radius of curvature of 1,500 mm to 10,000 mm, preferably of 2,000 mm to 5,000 mm.
At least one forming suction box includes at least one suction chamber, whose vacuum is adjustable/controllable by way of a controllable vacuum source. This permits, and even enhances considerably the adjustment of optimum operating conditions in the area of the forming suction box.
In order to once more increase the dewatering capacity of the twin wire former in accordance with the present invention, while maintaining good web qualities, a multitude of forming strips are located opposite at least one forming suction box. In accordance with the present invention at least one of the forming strips is mounted flexibly and/or at least one of the forming strips is mounted stationary, whereby their base position is adjustable relative to their wire, for example by way of sliding or pivoting.
Additionally, at least one wet suction box is located downstream from at least one forming suction box as viewed in the direction of wire travel. Preferably, the wet suction box is supplied with vacuum, whereby the vacuum is adjustable/controllable by way of a controllable vacuum source. This permits, and even considerably enhances, the adjustment of optimum operating conditions in the area of the wet suction box.
In order to keep the spatial dimensions of the twin wire former according to the present invention as small as possible, a turning roller is located prior to the separation element as viewed in the direction of wire travel, thereby reducing the actual horizontal and/or vertical length of the twin wire zone to a certain degree.
In order to permit further processing of the fiber web that is supported on the wire after the separation from the top and bottom wires, at least one flat suction box and a suction couch roll are located after the separating element as viewed in the direction of travel of the wire. This allows the degree of dewatering of the fiber web to be increased further.
When using wood-free fiber suspensions it is also advantageous if at least one machine-wide separating element, specifically a plate, is located in the nozzle of the headbox.
In a first embodiment, the twin wire zone of the twin wire former according to the present invention can essentially rise vertically from the bottom to the top, preferably with a vertical excursion of xe2x88x9215xc2x0 to +15xc2x0, preferably from xe2x88x925xc2x0 to +5xc2x0; and in a second configuration can rise from the bottom to the top with an incline from the horizontal plane of approximately 5xc2x0 to 45xc2x0. In another embodiment, the twin wire zone can slope from the top to the bottom with sloping gradient in the end zone. These embodiments represent the known possibilities in accordance with the state of the art, and have proven themselves frequently in the field.
It is understood that the aforementioned characteristics of the invention, which will be explained in further detail below, may be utilized not only in the cited combinations but also in other combinations, or freestanding, without abandoning the scope of the invention.